Efforts during the current reporting period were focused on Aim 3, evaluate SH2 domain antagonists as anti-cancer drugs. Specifically, we sought combinations of Grb2 SH2 domain antagonists and anti-cancer agents currently in use where known signaling pathways indicated the possibility of complementation, enhancement and/or synergy in anti-tumor efficacy. Grb2 was found by others to strongly promote cell cycle progression and proliferation downstream of the BCR-ABL oncogene, warranting further investigation of Grb2 SH2 domain binding antagonists in combination with imatinib mesylate, a prevalent current therapy for chronic myelogenous leukemia (CML) using cultured cell model systems. CML is characterized by the frequent and abundant presence of Bcr-Abl oncoprotein, a constitutively activated form of the Abl tyrosine kinase that has been linked to malignant transformation. The BCR-ABL fusion gene typically arises from a t(9;22)(q34;q11) translocation in bone marrow progenitor cells that produces a shortened chromosome 22, known as the Philadelphia chromosome. Imatinib is an oral Bcr-Abl kinase inhibitor and a current first line therapy for CML. Although impressive therapeutic responses have been achieved in the treatment of CML in the chronic phase, response rates in patients with more advanced disease are lower, and these responses are generally transient. Moreover, emerging evidence indicates that a significant proportion of patients in all phases of the disease fail to achieve an optimal response to imatinib due to innate or acquired resistance. The available treatments for patients with imatinib-resistant disease are extremely limited and there is an urgent need for improved treatment options for these patients. The underlying mechanisms of imatinib resistance include BCR-ABL gene rearrangement (the most common mechanism), overabundance of Bcl-Abl, and activation of Bcr-Abl-independent pathways, such as signaling through members of the Src family of kinases. In patients with Philadelphia chromosome positive CML, phosphorylation of Y177 in the BCR region of the Bcr-Abl oncoprotein enables binding to the Grb2 SH2 domain and thereby the activation of several downstream signaling pathways, including the Ras/MAPK pathway, which controls cell growth and proliferation. Furthermore, Grb2 mutants missing one of its two SH3 domains impair cell transformation by Bcr-Abl, highlighting the importance of Grb2 in this pathway and suggesting that Grb2 SH2 domain binding antagonists may provide an effective alterative or adjunct therapeutic strategy for CML patients resistant to imatinib. We found that the small molecule Grb2 SH2 domain binding antagonist TB03 retarded the growth of the CML-derived cell line K562 and synergistically enhanced growth suppression by imatinib. Moreover, imatinib suppressed a subpopulation of K562 stem cells, identified by the stem cell marker aldehyde dehydrogenase, and TB03 enhanced this activity. The combination also inhibited secretion of two other activators of the Ras/MAPK pathway: epidermal growth factor, the upregulation of which has been associated with imatinib resistance, and basic fibroblast growth factor, shown to be elevated in the blood of CML patients. TB03 inhibited the binding of tyrosyl phosphorylated proteins, including phospho-Bcr-Abl, to the Grb2 SH2 domain, significantly reduced tyrosyl phosphorylation of intracellular proteins and enhanced this activity of imatinib. TB03 with imatinib also increased expression of the tumor suppressor hsa-miR-34b in K562 cells. These findings suggest that targeting both Bcl-Abl and Grb2 may delay acquired imatinib resistance and more effectively control leukemia progression.